1. Field of the Invention
This invention relates generally to the field of carbon fiber reinforced metal matrix composites and specifically to fiber coatings that enhance wettability without degradation when immersed in molten metal.
2. Prior Art
In the past, a basic problem with carbon-fiber-reinforced metal matrix composites has been that the carbon or graphite-fibers resist wetting when immersed in the molten metal baths used to form the metal matrix. As a result, the fibers would have to be coated with a film that facilitated wetting and, in addition, also protected the fibers against chemical degradation during process and use. The currently used process relies upon the chemical vapor deposition of a thin film of titanium and boron to facilitate wetting as described in U.S. Pat. Nos. 4,223,075 of Sept. 16, 1980 to Harrigan, Jr. et al, 3,860,443 of Jan. 14, 1975 to Lachman et al, and 4,082,864 of Apr. 4, 1978 to Kendall et al. Although meritorious in concept, it is relatively expensive and inconsistent as to results.
As an improvised and novel alternative to the supra chemical vapor deposition, the recently invented process as noted supra by the same inventor entitled, "Carbon-Reinforced Metal-Matrix Composites" filed on Aug. 26, 1981 and having U.S. Ser. No. 296,957, uses a relatively thin metal-oxide coating that is deposited on the fiber surfaces by passing the fiber bundles through an organometallic solution followed by hydrolysis or pyrolysis of the organometallic compound to yield the desired coating. The oxide-coated fibers are readily wetted by a molten metal. The above mentioned metal-oxide technique yields improved results for carbon or graphite fibers having relatively high strength and low modulus such as T300 graphite fiber produced by Union Carbide Corp. which is made from polyacrylonitrile (PAN) precursor which has a stiffness of approximately 35.times.10.sup.6 psi. Recently, carbon fibers having relatively high moduluses, such as P100 graphite fiber produced by Union Carbide Corp. which is made from mesophase pitch which has a stiffness of approximately 100.times.10.sup.6 psi, have been fabricated. These relatively high modulus P100 fibers having a surface metal-oxide coating when immersed in a molten metal such as magnesium have been found to have relatively very little magnesium adhered to the fibers. Scanning Auger Microprobe (SAM) analysis reveals that immersion in liquid magnesium causes the metal-oxide coating to separate from the fibers, indicating that the metal-oxide coating does not adhere to the P100 fibers as well as to the T300 fibers. The difference in adhesion to the two fibers is due to the difference in both the surface morphology and chemical reactivity of the two fibers. The T300 fiber surfaces are rougher and more porous than the P100 surfaces. The P100 relatively high modulus fibers are more graphitic and this results in a smoother more chemically inert and less adhesive surface for the coating. Accordingly, there existed a need for treating relatively high modulus fibers so as to improve the adhesion of the metal oxide coating thereto when immersed in a molten metal.